Stock diversification is considered to be a change in the uniformity of stock ownership and will be a fundamental and influential factor in predicting future cash flows. Management styles and theories have undergone many changes and developments in the course of their evolution, organizations, human resources and material resources. These changes and developments have attracted the attention of managers to stock diversification, and it is important to examine the causes and factors affecting it. Therefore, the purpose of this study is to examine the relationship between investment opportunities and stock diversification with the moderating role of auditor rotation. The statistical population of the study is companies listed on the Tehran Stock Exchange and the sample under study includes 165 companies listed during the years 2020 to 2024. The research method is descriptive and in terms of the relationship between variables is causal-correlation, and it is applied in terms of purpose. The regression method and panel data as well as the fixed effects model have been used to process and test the hypotheses. The results of the hypothesis analysis showed that investment opportunities have a negative and significant relationship with stock diversification, and competition in the product market has a positive and significant effect on the relationship between investment opportunities and stock diversification. According to the research results, it is recommended that investors and capital market activists, in addition to financial variables, pay attention to variables such as product market competition when making investment decisions and include them as factors affecting the diversification of companies' stocks in their decision-making models.